This invention relates generally to the manufacture of catheters, and more particularly to the manufacture of catheters having rotatable operative elements.
Currently, there exist rotating element catheters, which can be used by physicians to provide a diagnostic or therapeutic effect within the body tissue of a patient, e.g., ultrasonic imaging or artherectomy. A typical rotating element catheter includes a flexible drive cable that extends the length of the catheter body, terminating proximally in a motor drive unit. An operative element, e.g., an ultrasonic transducer or artherectomy blade, is distally mounted to the drive cable. Operation of the drive unit rotates the drive cable, which, in turn, rotates the operative element at high speeds to produce the desired diagnostic or therapeutic effect. Due to the nature of placing indiscriminately rotating elements inside a patient, there is always a risk that the rotating element could inadvertently damage tissue if the catheter is defective or mishandled.
For example, some ultrasonic imaging catheters can provide two-dimensional 360xc2x0 images along the length of a blood vessel by rotating an ultrasonic transducer at high speeds, while linearly moving the ultrasonic transducer in the distal direction relative to the catheter member. If the distal end of the catheter member is kinked, or otherwise formed into a tight curve, there exists the possibility, however so slight, that the rotating ultrasonic transducer could perforate through the catheter member and damage the surrounding tissue. This is caused, in part, by the fact that the drive unit is designed to maintain the speed of the transducer at a set level, accordingly increasing or decreasing the torque that is applied to the drive cable. In doing so, the drive unit does not discriminate between normal frictional loads, i.e., frictional loads caused by normal friction between the drive cable and catheter member, and abnormal friction loads, i.e., frictional loads caused by an abnormal circumstance, e.g., the boring of the transducer through the wall of the catheter member.
As a precaution, these types of ultrasonic imaging catheters are designed, such that the drive shaft fails if the torque required to rotate the ultrasonic transducer becomes too great. This design contemplates providing a circumferential space between the drive cable and the catheter member along a portion of the catheter, allowing the drive cable to wind or ball up within the space when the torque applied to the drive cable exceeds a critical magnitude. Presumably, such an excess in force will occur if the rotating ultrasonic transducer begins to perforate the catheter member, resulting in a failed drive cable, and preventing the ultrasonic transducer from further boring through the catheter member.
Typically, however, the drive shaft fails, not because the ultrasonic transducer is boring through the catheter member, but rather because the drive cable is subjected to excessive frictional forces. Such forces are often a result of having to route the catheter through the tortuous vasculature of a patient, forcing the drive cable to rotate through many curves. Any mishandling of the catheter while operating the motor drive unit, e.g., overtightening the touhy-borst valve through which the catheter is introduced into the patient, exacerbates this situation. Because the drive unit is designed to maintain the rotation of the ultrasonic transducer at a uniform speed, the motor drive unit increases the torque that is applied to the drive cable to compensate for any increase in frictional force, thereby risking failure of the drive cable. In fact, of all the failed ultrasonic imaging catheters returned to the assignee of this application, approximately seventy percent fail as a result of this phenomenon.
There thus remains a need to prevent premature failure of a drive cable within a catheter, while minimizing the potential risk of inadvertently damaging tissue by the rotating operative element distally mounted on the drive cable.
The present inventions are broadly directed to rotating element catheters and catheter assemblies that employ springs to prevent rotational energy from being transmitted from a motor drive unit to the catheter element under defined circumstances.
In accordance with a first aspect of the present inventions, a catheter assembly includes an elongate member in which there is disposed a rotatable catheter drive shaft, e.g., a flexible drive cable. The catheter drive shaft may have an operative element, e.g., an ultrasonic transducer or an artherectomy blade, distally mounted thereon for providing diagnostic or therapeutic functions to the physician. In the case of ultrasonic imaging, the elongate member can take the form of a telescoping guide sheath slidably disposed about an imaging core (i.e., the catheter drive shaft and ultrasonic transducer) to provide the physician with two-dimensional 360xc2x0 ultrasonic images of surrounding body tissue.
To control the rotation of the catheter drive shaft, the catheter assembly includes a driver member and a driven member. The driven member is mechanically coupled (either directly or indirectly) to the proximal end of the catheter drive shaft. One of the driven and driver members comprises a spring (e.g., a coil spring or watch spring) that cooperates with other of the driven and driver members, such that the driven and driver members are rotatably engaged with each other before the applied torque exceeds a critical magnitude, and rotatably unengaged with each other after the applied torque exceeds the critical magnitude. In the preferred embodiment, the spring is interference fitted with the other of the driven and driver members, in which case, the spring can be advantageously wound such that the interference fit decreases in the presence of the applied torque. The driven and driver members are preferably located entirely within the catheter, e.g., in a proximal hub configured to interface with a motor drive unit, but a portion of the entirety of the driven and driver members can be located elsewhere, e.g., in the motor drive unit.
In accordance with a second aspect of the present inventions, the driver member comprises a motor drive shaft. The driven member comprises a coil spring, which is configured to cooperate with the motor drive shaft (e.g., by interference fitting) when the catheter is mated with the motor drive unit, such that the coil spring and motor drive shaft are rotatably engaged with each other before the applied torque exceeds a critical magnitude, and rotatably unengaged with each other after the applied torque exceeds the critical magnitude. In the preferred embodiment, the coil spring can be fixably disposed within a receptacle formed within the driven member, such that the coil spring is interference fitted over the motor drive shaft when received into the receptacle.
In accordance with a third aspect of the present inventions, the driver member comprises a motor drive shaft with a mounted watch spring. The driven member is configured to cooperate with the watch spring (e.g., by interference fitting) when the catheter is mated with the motor drive unit, such that the watch spring and driven member are rotatably engaged with each other before the applied torque exceeds a critical magnitude, and rotatably unengaged with each other after the applied torque exceeds the critical magnitude. In the preferred embodiment, the driven member includes a receptacle for receiving the watch spring.
In accordance with a fourth aspect of the present inventions, a coil spring is interference fitted over a rigid cylindrical member, wherein one of a driver member and a driven member includes the coil spring, and the other of the driver member and driven member includes the rigid cylindrical member. In the preferred embodiment, the member that includes the coil spring includes another rigid cylindrical member to which one end of the coil spring is affixed.
In accordance with a fifth aspect of the present inventions, a watch spring is interference fitted within the cylindrical cavity of a rigid receptacle, wherein one of a driver member and a driven member includes the watch spring, and the other of the driver member and driven member includes the rigid receptacle. In the preferred embodiment, the member that includes the watch includes a rigid member to which one end of the watch spring is wound around and affixed.
Other and further objects, features, aspects, and advantages of the present invention will become better understood with the following detailed description of the accompanying drawings.